This invention relates to synthetic resin lens having relative high refractive indices, and more particularly to such synthetic resin lens having an antireflection coating including a hard coat layer and an antireflection layer to reduce reflection occurring at the interface between the lens and hard coat layer.
In recent years, lenses made from synthetic resin materials, such as diethylene glycol bisallycarbonate and polymethyl methacrylate have found wide acceptance in place of lenses formed of inorganic glass. Such lenses are used particularly for spectacles as lenses of synthetic resin are significantly less breakable than glass lenses. Accordingly, they have been favorably accepted as a matter of safety. More recently, lenses of reduced thickness using resins having higher refractive indices have been developed. The appearance of these latter lenses has eliminated the disadvantage previously found with synthetic resin lenses. This shortcoming related to the inevitability of having a lens of greater thickness than that of an organic glass.
Synthetic resin lenses are also popular where the lightweight of the lens is of interest. This characteristic makes a synthetic lens outstanding for use as a spectacle lens. Over the past ten years there has been a sharp increase in the number of persons who prefer spectacle utilizing synthetic resin lenses. These lenses are by far lighter in weight than organic glass lenses and provide a higher degree of wearing comfort. As noted above, such synthetic lenses for sight correction are formed mainly of diethylene glycol bisallycarbonate. This resin has a relatively high degree of mar resistance and is relatively easy to dye. A lens of this resin has a greater center and edge thickness than an inorganic glass lens. In particular, users tend to complain about a strong minus lens having too large a shingle thickness.
The advantages of the synthetic resin lenses noted above account for the fact that they are rapidly taking the place of inorganic glass lenses. Even though the synthetic lenses do enjoy the high impact resistance and light weight, they do tend to scratch or scar more readily than inorganic glass lenses. In an effort to overcome this shortcoming, the practice has been adopted of depositing a hard coat film of an organic or inorganic substance on the surface of a synthetic resin lens. Particularly in the case of spectacle lenses, the synthetic resin lenses are coated with antireflection coatings which serve as a hard coat. Such lenses are currently available commercially. These antireflection coatings generally have a film construction as illustrated in FIG. 1 and include a hard coat layer 11 and an antireflection layer 12 superimposed on the surface of a lens 10 made of synthetic resin.
Hard coat layer 11, more often than not, is formed of silicon dioxide. This is done because silicon dioxide is capable of quickly forming a film of good quality when vacuum deposited by use of electron beams. Silicon dioxide has a refractive index between 1.44 to 1.46, which is low compared with the refractive indices of lenses formed from ordinary synthetic resins. When antireflection layer 12 is superimposed on hard coat layer 11 of silicon dioxide, hard coat layer 11 has an adverse effect upon the properties of antireflection layer 12. Specifically, reflection occurs at the interface between lens 10 and hard coat layer 11 due to the difference between the refractive indices of the two adjacent materials. This results in an interference wave which forms overlaps in the spectral reflectance property of antireflection layer 12.
This spectral reflectance property including a ripple denoted by 21 is shown in FIG. 2. The size of ripple 21 increases in proportion to the difference between the refractive index lens 10 and that of hard coat layer 11 increases. When hard coat layer 11 is formed of silicon dioxide which has a refractive index of about 1.46 and lens 10 is made of a synthetic resin having a high refractive index of about 1.60, for example, the difference between the refractive indices is 0.14. Consequently, the height of ripple 21 approaches 2%. Attempts to lower the reflectance on one surface of lens 10 to less than 2% by means of antireflection film 12 have not been successful.
One possible solution to this problem may lie in using dielectric substances for the hard coat layer which have the same refractive index as the synthetic resin lens. The use of such dielectric substance would overcome reflection at the interface between hard coat layer 11 and synthetic resin lens 10, and consequently eliminate ripple 21. In actual practice, however, there is no known dielectric substance which has the same refractive index as the synthetic resin lens and the ability to form quickly a vacuum-deposited film of acceptable quality.
Accordingly, it is desirable to provide a lens made of a synthetic resin material having a antireflection coating including a hard coat layer which overcomes these problems of the prior art. Further, such lenses should exhibit outstanding antireflection properties and be able to be formed by any of the conventional film-forming techniques, such as vacuum-deposition and spattering on a synthetic resin lens having a varying refractive index.